The present invention relates to wireless systems and, more particularly, to a method and apparatus for performing analog mode operations in a wireless TDMA system, wherein in-phase (I) and quadrature (Q) values are used to represent audio information, data and signaling tones in the analog mode of operation.
FIG. 1 illustrates a wireless system 1. The wireless system 1 comprises a plurality of cells 2, each of which comprises a transceiver 3 that is electrically coupled to an antenna 4. Each transceiver 3 and its respective antenna 4 together comprise a base station. In wireless systems that utilize the well-known time division multiple access (TDMA) communications protocol, digital voice and data are transmitted using I, Q pairs. The I, Q pairs are modulated/demodulated using a modulation/demodulation technique known as quadrature phase shift keying (QPSK). This is typically viewed as a TDMA mode of operation because the I, Q pairs are used to produce bit transitions that represent the voice and data. Therefore, this type of operation will be referred to hereinafter as the TDMA mode of operation.
Many TDMA systems in use today are also capable of operating in what is typically referred to as an analog mode of operation. In the analog mode, audio and data signals are transmitted by converting digital samples representative of the signals into analog signals and by FM modulating the analog signals for transmission over air. Therefore, the analog mode utilizes FM modulation as opposed to the QPSK modulation technique utilized in the TDMA mode. The standards for implementing the analog mode and the TDMA mode in a TDMA system are set forth in the Air Interface Standard, IS-138.
It is known in TDMA systems to utilize one set of hardware components in the transceiver front end for TDMA mode operations and another set of hardware components for analog mode operations. This is because data and voice are normally represented by I, Q pairs in the TDMA mode and by digital samples of the analog waveform in the analog mode. Also, separate hardware has been utilized for performing QPSK modulation and FM modulation. Of course, utilizing separate sets of hardware components increases the complexity of the transceiver as well as the costs associated with these components. It would be desirable to provide a transceiver that utilizes a single set of hardware components for TDMA mode and analog mode operations. Utilizing a single set of hardware components for both modes could reduce the costs and complexity of the transceiver.
FIG. 2 represents the components of a known transceiver of a wireless TDMA system for operating in the analog mode. The hardware components that are utilized in the transceiver for TDMA mode operations are not shown in FIG. 2 for ease of illustration. The hardware components that are utilized for the analog mode include the encoder 12, the decoder 13, the digital-to-analog converter (DAC) 14, the analog-to-digital converter (ADC) 15, the ADC 18, the DAC 19, the FM demodulation hardware 20 and the FM modulation hardware 21.
In order to transmit audio signals in the analog mode, a digital signal processor (DSP) 22 outputs a digital representation of an analog voice signal to the DAC 19, which converts the digital representation into an analog waveform and outputs the analog waveform to the FM modulation hardware 21. The FM modulation hardware 21 frequency modulates the analog waveform for transmission over air. When an FM modulated audio signal is received by the transceiver 10, the FM demodulation hardware 20 demodulates the analog audio signal and delivers the demodulated signal to the ADC 18, which converts the analog audio signal into a digital representation of the analog audio signal. The digital representation of the analog audio signal is then delivered to the DSP 22, which processes the digital signal in accordance with various software routines and delivers the processed digital signal to other components of the base station for routing to the mobile telephone switch office (MTSO).
When a data signal is to be transmitted by the base station to a mobile unit (not shown) using the analog mode, the main controller 23 delivers a digital representation of the data signal to the encoder 12, which encodes each bit of the digital data into a digital representation of the corresponding analog waveform. The encoded signal is then delivered to the DAC 14. The DAC 14 converts the encoded signal into an analog data signal, which is then delivered to the FM modulation hardware 21. The FM modulation hardware 21 FM modulates the signal for transmission over air.
When a data signal is received by the transceiver 10 when it is operating in the analog mode, the FM demodulation hardware 20 demodulates the RF signal, which is then converted by the ADC 15 into a digital signal. The digital signal is then delivered to the decoder 13, which decodes the digital signal into a digital message that is usable by the main controller 23. The decoder 13 then delivers the digital message to the main controller 23, which processes the digital message to extract the content.
As stated above, separate hardware components are used for performing TDMA mode and analog mode operations in TDMA systems. It would be advantageous to provide a transceiver that could perform all of these functions without the need for separate hardware components. Eliminating certain hardware components may reduce the complexity of the transceiver and the costs associated with the transceiver. Accordingly, a need exists for a method and apparatus for use in a transceiver that enable the same hardware components to be used for TDMA and analog mode operations.
The present invention provides a method and apparatus for use in a transceiver of a wireless system that enable analog mode operations to be performed using in-phase (I) and quadrature (Q) values. Since TDMA mode operations are normally performed using I and Q values, the method and apparatus of the present invention enable, but do not require, the same hardware components to be utilized for TDMA and analog mode operations. The apparatus of the present invention comprises a processor, such as, for example, a digital signal processor (DSP), that performs FM modulation to generate I, Q pairs when the transceiver is operating in the analog mode. The processor also performs the I, Q encoding and decoding operations that are normally performed in hardware in transceivers operating in the TDMA mode.
When transmitting in the analog mode, the processor encodes and FM modulates a digital representation of the signal to be transmitted into I, Q pairs and outputs the I, Q pairs to a digital-to-analog converter (DAC) comprised by the apparatus. The DAC converts the digital I, Q pairs into analog signals and outputs the analog signals to an I, Q cosine wave generator. The cosine wave generator generates an in-phase cosine wave and a quadrature phase cosine wave having amplitudes that are proportional to the I and Q values, respectively. The cosine waves are summed for transmission over air.
When receiving audio or data in the analog mode, a frequency converter converts the received radio frequency (RF) signal down to an intermediate frequency (IF). A digital down converter converts the received signal into digital I and Q values and outputs them to the processor. The processor then decodes the I and Q values into a digital representation of the received signal to extract the signal content.
When a data signal is received by the digital down converter, the data generally is in one of two formats. In one of these formats, a data message begins with an initial 64-bit dotting sequence, which is followed by radio link words (RLWs). Each RLW includes a 37-bit Dotting sequence followed by an 11-bit Barker sequence, which is followed by 48 bits of data and a check sum. Each RLW is repeated five times. Therefore, the DSP has five opportunities to decode each RLW. The Dotting sequence is used for clock recovery and the Barker sequence is used to determine where the first bit of data begins.
The Dotting sequences are sequences of 1s and 0s that produce a 5 kilohertz (kHz) waveform, as required by the aforementioned Air Interface Standard. The processor utilizes the initial Dotting sequence to determine that a data message is being received, and thereafter utilizes the Dotting sequence of each RLW to help detect each RLW. The processor detects the Dotting sequences by measuring the level of 5 kHz energy received. If the level of 5 kHz energy exceeds a certain threshold level, the processor determines that the Dotting sequence has been detected and that a data message is being received. The level of 5 kHz energy is measured by performing a frequency domain analysis, preferably by taking the Fourier Transform of the received signal. Once the Dotting sequence has been detected, the processor determines the phase of the 5 kHz signal, which synchronizes the processor to the start of each data bit. The processor then begins looking for the Barker sequence.
Once the processor has detected the Dotting sequence and has performed clock recovery, the processor has determined that it has located the Dotting sequence, but has not yet determined where it is in the Dotting sequence. The processor uses the Barker sequence to determine where the data begins. The Barker sequence is a specific sequence of 1s and 0s. The processor looks for this specific sequence and, when it has been detected, determines that the next bit is the first bit of data. Therefore, the processor utilizes the Barker sequence to align itself with the data.
In order to decode the data, the processor evaluates the bits of a particular number of repeats of an RLW before determining whether any bit represents a binary 1 or a binary 0. For each repeat, the processor evaluates each bit to determine whether it is likely to be a binary 1 or a binary 0. Once the bits have been evaluated for a particular number of repeats, the processor uses the results of all of the evaluations to determine whether each bit is a binary 1 or a binary 0. In accordance with the preferred embodiment, the processor looks at each bit of the data for each RLW of the five repeats and generates and stores a confidence factor associated with each bit. The confidence factor is based on how closely the waveform represents a 1 or a 0 and the signal strength. For example, if the waveform is relatively noisy, or if the RF signal strength is low, the processor assigns less weight to its determination as to whether the waveform represents a 1 or a 0. If the waveform has relatively little noise and if the RF signal strength is relatively high, the processor assigns more weight to its determination as to whether the waveform represents a 1 or a 0.
Once this process has been performed for each repeat of the current RLW, the processor takes the sum of the confidence factors associated with each bit for each RLW for all five repeats and compares the sum to a threshold level, which preferably is 0. If the sum is eater than the threshold level, the processor determines that the bit is a 1. If the sum is less than the threshold level, the processor determines that the bit is a 0. Therefore, by the e nd of the five repeats of the RLWs, the processor has determined whether each bit of data is a 1 or a 0.
This process is also performed for the Dotting and the Barker sequences. For each of the five repeats, the processor obtains a confidence factor associated with each bit of the combined Dotting and Barker sequences in the aforementioned manner. Therefore, by the end of the five repeats, the processor has determined whether each bit of the combined sequence is a 1 or a 0. Since the Dotting and Barker sequences are known, the processor compares the results of the determination to what the combined sequence is supposed to be and determines the number of bit errors that have occurred. If the number of bit errors exceeds a predetermined threshold, the processor determines that the entire message is invalid. The processor could instead use only the Barker sequence to make this determination, but using both the Barker and the Dotting sequences provides better results.
The present invention also provides a signaling tone detection routine that is used to distinguish between when data is being received and when signaling tone is being received. Signaling tone of various lengths is transmitted from the mobile unit to the base station to communicate various types of information to the base station.